On Fri, 23 May 2008 17:29:48 -0700 (PDT), Frank Krygowski
<
[email protected]> wrote:
>On May 23, 4:34 pm, [email protected] wrote:
>>
>>
>> Frank wrote about how dropping from second to first in a 5-speed
>> Sturmey-Archer seemed to reveal a dramatic difference in efficiency.
>
>Well, what I really felt was "Wow. That downshift didn't seem to help
>much at all. I'm working just as hard even though I'm climbing
>slower."
>
>The situation: Bike was an ancient ladies frame Dunelt, owned by a
>widow friend. It had been her husbands. (He'd had a much larger rear
>cog brazed onto the original.)
>
>We were visiting on vacation, and I was attempting to give the bike a
>little tuneup for her. Their town, in the Appalachian foothills,
>featured lots of short steep climbs (hence the big rear cog). She'd
>warned me that her husband had some trouble keeping the hub gear
>properly adjusted, and it demonstrated some trouble with a false
>neutral. I fussed with cable adjustment and improved things, but
>never got it to shift perfectly.
>
>I've reassembled S-A three speeds, but never a five speed. I'm not
>familiar enough with the guts to know if some bad component in there
>could generate both shifting problems and extra inefficiency in low
>gear.
>
>This was long ago (probably 15 years), so my memory isn't to be
>trusted; but IIRC, I read about the inefficient low _after_ my
>experience with that bike, so it was a case of the reading confirming
>my impression, not vice-versa.
>
>- Frank Krygowski
Dear Frank,
I kept looking and found these extended comments, which support and
explain your impression of 1st gear in a 5-speed Sturmey-Archer being
less efficient than 2nd gear. The actual claim from S-A is a drop from
93% to 89%, but that might mean as much as 93.4% to 88.5%.
Just to confuse things, there were two versions of the 5-speed S-A
hub.
***
David Henshaw, Oct 98:
The 5-speed Sturmey is less efficient than the 3-speed in gears 1 and
5. Try riding up the same hill on a 3-speed in gear 1 and a 5-speed in
gear 1. The lower gearing on the 5 gives no real advantage. I was
sceptical until Andrew Ritchie, the man himself, demonstrated this.
Now I believe it. And the 5 is very adjustment-sensitive, unlike the
3, which lasts forever and ever.
On a 5-speed, I'd go for a 14-tooth sprocket, which gives about 8%
lower gearing. But, please remember to change the shim pack - the 14T
is thicker than the 13T... The 12% optional lower gearing is certainly
worth fitting on the 5-speed if you want really low gearing, but I'm
not sure about the 18% - a front changer is probably a more sensible
option.
I'm still happiest with a light, efficient 3-speed with standard
gearing. My clunky old bike still holds the unofficial Brompton speed
record, set at CycleFest 1996 by Richard Grigsby.
***
Willi Mindak, Nov 98:
I received an e- mail today from Marketing at SA. This was in reply to
an e- mail I send over a month ago. It says:
Dear Sir
The only information I have on this type relates to the 5 speed hub,
the information is as follows:-
Efficiency
Gear Ratios Efficiency
Super Low 0.667 89%
Low 0.789 93%
Normal 1.00 96%
High 1.266 94%
Super High 1.5 88%
Efficiency Test Conditions
Motor Torque Arm balance weight = 2.2Kg
Motor Speed = 64 RPM
No. of chainwheel teeth = 46
No. of Sprocket teeth = 18
I hope this information assists you.
Yours faithfully
Trevor Wilkinson
Marketing Executive
No surprises here. Legend had it that 1st and 5th gear were a bit
harder to work. Third gear is best with no internal ratio; I take it
the 4% are just bearing/ chain losses. I also asked for the 3 speed
values, but no joy. Anyway, these values are in the expected range.
They are actually better than I thought.
***
Why is highest and lowest gear in a 5 speed less efficient than
highest and lowest in a 3 speed?
Stein Somers, Oct 1998:
My guess: in the extremer gears, the planets in the cage roll on a
bigger central fixed cog, hence they are smaller in diameter and make
more rounds per minute. Both factors increase the friction at their
(simple) bearings. Maybe on top more than three planets are needed to
cope with the driving forces because the teeth on a smaller planet
make less firm contact with the central cog.
Come to think of it, the basic planetary system can only reach a 2:1
drive ratio using infinitely small planets. That's a 400% gear ratio,
low to high gear. (SA-3 has 178%, SA-5 has 225%). So how does the
illustrious Rohloff hub obtain more than 500%??
Carsten Thies, Jan 2003:
It has two identical planetary gears in line, giving 7 gears*, plus an
additional step-down planetary gear for the lower 7 gears.
* it's not 3x3=9 gears because step-up step-down or vice versa would
give the same as direct gear.
Custfold, Oct 1998:
The reason for the extreme high/extreme low being accessed by pulling
the second lever should give a clue - the old SA5 system used a second
cable to pull the dog over on the LHS of the unit and engage a step-up
ratio in the drive chain - otherwise the middle 3 were direct into the
gear cage. This in turn means a bigger slot in the axle and extra
little piece to slide in it for all 5 speed hubs. The Sprinter merely
uses a longer pull stroke to operate a 2-stage little bit in a slot
from 1 cable.
***
Everything above is from:
http://stein.dommel.be/brompton/chapters/Gears.html#HubGearEfficiency
Cheers,
Carl Fogel